The genome of hepatitis C virus (HCV), the etiologic agent of most post-transfusion non-A, non-B cases, is a positive stranded RNA virus. The susceptibility of chimpanzees has allowed collection of serial blood and liver biopsies from all stages of infection, yet no complete virions have been convincingly isolated. A likely reason for the failure to demonstrate these agent(s) or to detect virus-like antigens is the very limited degree of viral replication in the infected host, as suggested by low infectivity titer of most plasma. We anticipate that the development of a primary culture of liver cells might prove susceptible.